Chapter 17, Problem 17.1KSP

Which of the acids in Table 16.5 (page 732) can be used to prepare a buffer of pH 6.5? (Select all that apply.)

(a) hydrofluoric acid

(b) benzoic acid

(c) hydrocyanic acid

(d) phenol

(e) none

Interpretation Introduction

Interpretation:

From the given acids, the acid used to prepare buffer solution of $\text{pH}\text{=}\text{6}\text{.5}$ has to be identified.

Concept introduction:

• $\text{pH}\text{=}{\text{pK}}_{\text{a}}\text{+}\text{log}\frac{\text{[conjugate base]}}{\text{[acid]}}$ is Henderson-Hasselbalch equation.
• Buffer solution is defined as a solution that oppose changes in $\text{pH}$ while adding little amount of either an acid or a base.
• Buffer solution is a combination of a weak acid and its conjugate base or a weak base and its conjugate acid.

To identify: The acid used to prepare buffer solution of $\text{pH}\text{=}\text{6}\text{.5}$.

Answer to Problem 17.1KSP

The acid that can be used to prepare the buffer given in the problem statement is option (e)

Explanation of Solution

Reason for correct option(d)

The acid used to prepare buffer solution is identified below.

When the concentration of conjugate base and weak acid are same the $\text{pH}\text{=}\text{pKa}$

The $\text{pKa}$ of $\text{HF is 3}\text{.14}$

The $\text{pKa}$ of $\text{Benzoic acid is 4}\text{.19}$

The $\text{pKa}$ of $\text{HCN is 9}\text{.30}$

The $\text{pKa}$ of $\text{Phenol is 9}\text{.9}$

The $\text{pKa}$ value of given acids is nowhere near the $\text{pH}$ of buffer solution.

Hence, the correct option is (e).

Reason for incorrect options (b), (a), (c), (d)

The $\text{pKa}$ value of given acids does not have values close to $\text{pH}$.  Hence, none of the given acids can form buffer at given $\text{pH}$.

Hence, the incorrect options are (b), (a), (c), (d)

Conclusion

From the given acids, the acid used to prepare buffer solution of $\text{pH}\text{=}\text{6}\text{.5}$ was identified.